Effect of surfactants on the stability and solar thermal absorption characteristics of water-based nanofluids with multi-walled carbon nanotubes

•The effect of surfactants on suspension stability and absorption are reported.•The relation between absorption and suspensions stability of nanofluids is presented.•MWCNTs nanofluids with SDBS have a high extinction coefficient and suspension stability. This paper reports the effect of various surfactants on the suspension stability and the solar thermal absorption characteristics of water-based nanofluids containing multi-walled carbon nanotubes (MWCNTs) that can be used as working fluids for volumetric solar thermal receivers. The water-based MWCNT nanofluids were prepared using a two-step method with four commonly used surfactants: sodium dodecylbenzenesulfonate (SDBS), cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 (TX-100). The stability of the four surfactant-treated nanofluids was analyzed for over a month with an in-house developed laser transmission system. The effect of temperature on the stability of the nanofluid/surfactant mixtures was also examined. In addition, to identify the absorption characteristics of the four nanofluids, the spectral extinction coefficients were measured using an UV–Vis–NIR spectrophotometer. The absorbed sunlight fraction was calculated using the measured spectral extinction coefficient, which enabled an evaluation of the absorption characteristics of the nanofluids. The MWCNT nanofluids were clearly shown to enhance the absorption rate of solar thermal energy. The suspension stability and the absorption characteristics were also strongly affected by the type of surfactant. Moreover, using the absorbed sunlight fraction and suspension-stability factor, we experimentally show the relation between the absorption characteristics and suspension stability in nanofluids.